Instruments are used in numerous scientific and industrial applications to measure parameters of interest for the particular technology being employed. Various types of instruments are utilized to measure parameters such as fluid flow, pressure, level, temperature, velocity, acceleration, weight, mass, radiation level or humidity. Measurement of these parameters sometimes makes it necessary to locate the instrument in an environment which exposes it either continuously or intermittently to adverse environmental conditions, such as pressure, humidity, temperature, dust, abrasive particles, corrosive gas and/or radiation level. A known solution to this problem is to surround only the delicate parts of the instrument in a protective housing together with closure seals and other seals around electrical wires entering or leaving the housing; however, when instruments of this construction are tested in simulated adverse environments, they often fail or become unacceptably inaccurate. Another known solution is to install the principal portions of an instrument in a location remote from the parameter being measured so that they are in a more benign environment which the instrument can withstand. However, locating the instrument at a distance from the parameter under measurement has several disadvantages, including that the instrument can be less accurate for some type of measurements when positioned a substantial distance from the parameter being measured, and that the increased distance may add complexity to the instrument system. Another prior proposal to solve the problem involves placing instruments in protective containers that are bulky and provide extremely poor access to the instrument for normal activities such as calibration and maintenance. Consequently, a need exists for an enclosure structure that will protect instruments against adverse environments, having a relatively simple construction, and allow easy access to the instrument for calibration and maintenance.